Multi-Layer Overlay Metrology Target and Complimentary Overlay Metrology Measurement Systems

ABSTRACT

A system for measuring overlay from a multi-layer overlay target for use in imaging based metrology is disclosed. The system is configured for measuring overlay from a multi-layer overly target that includes three or more target structures, wherein a first target structure is disposed in a first process layer, a second target structure is disposed in a second process layer, and at least a third target structure is disposed in at least a third process layer. The system includes an illumination source configured to illuminate the target structures of the multi-layer overlay target, a detector configured to collect light reflected from the target structures, and one or more processors configured to execute a set of program instructions to determine overlay error between two or more structures based on the collected light from the plurality of targets.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is related to and claims benefit of the earliestavailable effective filing date from the following applications. Thepresent application constitutes a continuation patent application ofUnited States patent application entitled MULTI-LAYER OVERLAY METROLOGYTARGET AND COMPLIMENTARY OVERLAY METROLOGY MEASUREMENT SYSTEMS, namingDaniel Kandel, Vladimir Levinski, and Guy Cohen as inventors, filed Jul.19, 2011, application Ser. No. 13/186,144, which is a regular(non-provisional) patent application of United States Provisional patentapplication entitled MULTI-LAYER OVERLAY METROLOGY, naming DanielKandel, Vladimir Levinski, and Guy Cohen as inventors, filed Aug. 3,2010, Application Ser. No. 61/370,341. U.S. patent application Ser. No.13/186,144 and U.S. Provisional Patent Application No. 61/370,341 areincorporated by reference herein in their entirety.

TECHNICAL FIELD

The present invention generally relates to an overlay target used foroverlay metrology, and more particularly to a multi-layer target andcomplimentary metrology systems.

BACKGROUND

In a variety of manufacturing and production settings, there is a needto control alignment between various layers or within particular layersof a given sample. For example, in the context of semiconductorprocessing, semiconductor-based devices may be produced by fabricating aseries of layers on a substrate, some or all of the layers includingvarious structures. The relative position of these structures bothwithin a single layer and with respect to structures in other layers iscritical to the performance of the devices. The misalignment betweenvarious structures is known as overlay error.

The measurement of overlay error between successive patterned layers ona wafer is one of the most critical process control techniques used inthe manufacturing of integrated circuits and devices. Overlay accuracygenerally pertains to the determination of how accurately a firstpatterned layer aligns with respect to a second patterned layer disposedabove or below it and to the determination of how accurately a firstpattern aligns with respect to a second pattern disposed on the samelayer. Presently, overlay measurements are performed via test patternsthat are printed together with layers of the wafer. The images of thesetest patterns are captured via an imaging tool and an analysis algorithmis used to calculate the relative displacement of the patterns from thecaptured images. Such overlay metrology targets (or ‘marks’) generallycomprise features formed in two layers, the features configured toenable measurement of spatial displacement between features of thelayers (i.e., the overlay or displacement between layers). FIGS. 1Athrough 2B illustrate typical overlay targets of the prior art. FIGS. 1Aand 1B illustrate overlay targets having 180 degree and 90 degreerotational symmetry, respectively, about a center of symmetry. Moreover,the target structures of FIGS. 1A and 1B include pattern elements (e.g.,102 a through 108 b), which are individually invariant to 90 degreerotation. Due to the 90 degree invariance of the individual patternelements the pattern elements of targets 100 and 101 of FIGS. 1A and 1Bare suitable for both X-overlay and Y-overlay measurements.

FIGS. 2A and 2B illustrate targets 200 and 201 which display invarianceto a 90 degree and 180 degree rotation, respectively. In contrast toFIGS. 1A and 1B, the pattern elements (e.g., 202 a through 208 d)display only 180 degree rotational symmetry. As such, at least twoseparate orthogonally oriented pattern elements must be used in order tomeasure overlay in both the X- and Y-direction. For instance, thepattern elements 202 a, 204 a, 202 d, and 204 d may be used to measureoverlay in a first direction, while elements 202 b, 204 b, 204 c, and202 c may be used to measure overlay in a second direction orthogonal tothe first direction.

Although existing targets and target measurement systems are suitablefor many implementation contexts, it is contemplated herein that manyimprovements may be made. The invention described herein disclosestargets and apparatus for enabling improved metrology measurements

SUMMARY

A system for measuring overlay from a multilayer overlay target, inaccordance with one or more embodiments of the present disclosure. Inone embodiment, the system includes an illumination source configured toilluminate a plurality of target structures disposed on one or moreprocess layers of a semiconductor device. In another embodiment, theplurality of target structures include three or more target structures,the three or more target structures including a first target structure,a second target structure and at least a third target structure. Inanother embodiment, at least some of the target structures include a setof two or more pattern elements. In another embodiment, the three ormore target structures are configured to share a common center ofsymmetry upon alignment of the three or more target structures. Inanother embodiment, at least one of the target structures is invariantto 90 degree rotation about the common center of symmetry. In anotherembodiment, the first target structure is disposed in a first processlayer, wherein the second target structure is disposed in a secondprocess layer different from the first process layer, wherein at leastthe third target structure is disposed in at least a third processlayer, the at least a third process layer different from the firstprocess layer and the second process layer. In another embodiment, afirst particular pattern element of the set of two or more patternelements includes three or more sub-elements and at least a secondparticular pattern element of the set of two or more pattern elementsincludes three or more sub-elements. In another embodiment, at least oneof the three or more sub-elements of the first particular patternelement or the three or more sub-elements of the at least the secondparticular pattern element comprise a set of three or more parallel linestructures aligned along a selected direction and distributed along adirection orthogonal to the selected direction. In another embodiment,the system includes a detector configured to collect light reflectedfrom the plurality of target structures. In another embodiment, thesystem includes one or more processors configured to execute a set ofprogram instructions maintained on a non-transitory memory medium, theset of program instructions configured to cause the one or moreprocessors to determine overlay error between two or more structuresbased on the collected light from the plurality of targets.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory onlyand are not necessarily restrictive of the invention as claimed. Theaccompanying drawings, which are incorporated in and constitute a partof the specification, illustrate embodiments of the invention andtogether with the general description, serve to explain the principlesof the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The numerous advantages of the disclosure may be better understood bythose skilled in the art by reference to the accompanying figures inwhich:

FIG. 1A is a top plan view of an overlay target;

FIG. 1B is a top plan view of an overlay target;

FIG. 2A is a top plan view of an overlay target;

FIG. 2B is a top plan view of an overlay target;

FIG. 3 is a top plan view of a multi-layer overlay target, in accordancewith one embodiment of the present invention;

FIG. 4 is a top plan view of a multi-layer overlay target, in accordancewith one embodiment of the present invention;

FIG. 5A is a top plan view of a multi-layer overlay target, inaccordance with one embodiment of the present invention;

FIG. 5B is a top plan view of a multi-layer overlay target, inaccordance with one embodiment of the present invention;

FIG. 6 is a top plan view of a multi-layer overlay target, in accordancewith one embodiment of the present invention;

FIG. 7 is a top plan view of a multi-layer overlay target, in accordancewith one embodiment of the present invention;

FIG. 8 is a top plan view of a multi-layer overlay target printed in thepresence of dummy fill, in accordance with one embodiment of the presentinvention;

FIG. 9 is a top plan view of a multi-layer overlay target printed in thepresence of dummy fill, in accordance with one embodiment of the presentinvention;

FIG. 10 is a top plan view of a multi-layer overlay target printed inthe presence of dummy fill, in accordance with one embodiment of thepresent invention;

FIG. 11 is a block diagram view of a system suitable contrastenhancement of a multi-layer overlay metrology target;

FIG. 12 is a block diagram view of a system suitable contrastenhancement of a multi-layer overlay metrology target;

FIG. 13A is a schematic view of an illumination pupil structure suitablefor contrast enhancement, in accordance with one embodiment of thepresent invention;

FIG. 13B is a schematic view of an illumination pupil structure suitablefor contrast enhancement, in accordance with one embodiment of thepresent invention; and

FIG. 13C is a schematic view of an illumination pupil structure suitablefor contrast enhancement, in accordance with one embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the subject matter disclosed,which is illustrated in the accompanying drawings.

Referring generally to FIGS. 3 through 10, an overlay target suitablefor imaging based overlay metrology is described in accordance with thepresent disclosure. In a general sense, the overlay targets of thepresent invention may be used to determine overlay error between twosuccessive process layers of a semiconductor wafer. For example, anoverlay target may be utilized to measure the alignment of a firstsemiconductor layer with respect to a second semiconductor layer, wherethe second layer and the first layer are disposed successively.Additionally, an overlay target may be used to determine alignment errorbetween two structures formed on a common semiconductor layer via two ormore different processes (e.g., lithographic exposures). For example, anoverlay target may be utilized to measure the alignment of a firstpattern with respect to a second pattern, where the first pattern andthe second pattern are successive patterns formed on the samesemiconductor layer.

For instance, in a measurement utilizing two or more overlay targets, anoverlay target may be printed at a specific location on a first waferlayer and a second wafer layer, so that when the first and second layersare properly aligned the pattern elements of the first structure andsecond structure of the overlay target also align. When the first andsecond layers are ‘mis-registered,’ however, a relative shift betweenthe pattern elements of the first structure 102 and the second structure104 of a given thin overlay mark 100 exists, a shift that can bemeasured through a variety of techniques.

The structures and pattern elements described herein may be fabricatedusing any process known in the art suitable for semiconductor waferprocessing, such as, but not limited to, photolithographic, etching, anddeposition techniques. Methods for printing overlay targets and theircontained structures, pattern elements, and pattern sub-elements aredescribed generally in U.S. application Ser. No. 11/179,819 filed onFeb. 23, 2006, and is incorporated herein by reference.

FIG. 3 illustrates a top plan view of a six-layer overlay target 300suitable for imaging based metrology, in accordance with an exemplaryembodiment of the present invention. In one aspect, the overlay target300 may include three or more target structures. In another aspect oftarget 300, each of the target structures of the overlay target 300includes two or more pattern elements. Note that for the purposes ofthis disclosure texture patterns in FIG. 2 (and figures throughout thisdisclosure) are used to represent the different target structures of atarget, wherein pattern elements belonging to the same target structurehave the same texture. The texture patterns displayed in the variousfigures of the present disclosure should not be interpreted as limitingas the selected texture pattern is not representative of a structuralaspect of the associated pattern element, but is merely utilized torepresent pattern elements of the same target structure. By way ofexample, as shown in FIG. 3, the target 300 may include six targetstructures (each structure illustrated with a unique texture). Further,each of the six target structures of target 300 may include two patternelements. For instance, as shown in FIG. 3, a first structure mayinclude pattern elements 302 a and 302 b, a second structure may containpattern elements 304 a and 304 b, a third structure may include patternelements 306 a and 306 b, a fourth structure may include patternelements 308 a and 308 b, a fifth structure may include pattern elements310 a and 310 b, and a sixth structure may include pattern elements 312a and 312 b. More generally, a given structure of target 300 (i.e.,first, second, third, or up to an Nth structure) may contain from twopattern elements up to and including an Nth pattern element.

In another aspect of target 300 of the present invention, each of thetarget structures of target 300 are designed such that each is invariantto a 180 degree rotation about a common center of symmetry 110. Forexample, as shown in FIG. 3, upon rotating the target structures aboutthe common center of symmetry 110 by 180 degrees the top view image ofthe structures remains identical to the top view image of the structuresprior to rotation. Resultantly, it will be recognized by those skilledin the art that the overall target, consisting of the multipleindividual structures, is invariant to a 180 degree rotation about thecommon center of symmetry 110 when properly aligned. In one embodiment,as illustrated in FIG. 3, the two pattern elements of each structure maybe oriented at positions diagonally opposed to one another, resulting in180 degree rotational symmetry for the overlay target as a whole.

It is recognized herein that the utilization of an overlay target 300invariant to a 180 degree rotation about the common center of symmetry110 allows for the use of the target 300 in overlay metrology betweenmore than two layers. In this manner, overlay metrology measurements maybe performed utilizing any pair of the six target structures present inoverlay target 300. Moreover, due to the collocation of the center ofsymmetries of each structure of target 300, overlay metrologymeasurements may be acquired from all six structures in a single imagegrab.

It should be recognized that while a first structure and a secondstructure share a common center of symmetry by design when a first layerand a second layer are properly aligned, upon misalignment between afirst layer and a second layer, the first structure and the secondstructure shift with respect to one another. As a result ofmisalignment, the center of symmetry of a first structure and the centerof symmetry of a second structure will shift and the center ofsymmetries of the first structure and the second structure will nolonger coincide. It is recognized that this concept may be extended toall of the structures within a given target of the present invention. Itis the measurement of this shift between centers of symmetries ofvarious structures of a target 300 which enables the overlaymeasurement. Measurement techniques that may be used in the context ofthe target 300 described herein are described in U.S. application Ser.No. 11/830,782 filed on Jul. 30, 2007, and Ser. No. 11/179,819 filed onJul. 11, 2005, and are incorporated herein by reference.

In another aspect, each pattern element of each structure of the target300 possesses an individual center of symmetry 110. Moreover, thepattern elements of target 300 are designed such that each patternelement (e.g., 302 a-302 b, 304 a-304 b and etc.) are invariant to a 90°rotation about the center of symmetry 110 of the individual patternelement. As a result of the 4-fold rotational symmetry of each of thepattern elements of each of the structures of the target 300, X-overlayand Y-overlay measurements may be performed utilizing the same patternelement.

It should be recognized by those skilled in the art that the number oftarget structures and the number of pattern elements within the targetstructures as depicted in FIG. 3 do not represent limitations, butrather should be interpreted as illustrative in nature.

Moreover, it will be recognized by those skilled in the art that the useof a rectangular target region, as depicted in FIG. 3, is not alimitation and that generally a variety of mark region shapes (e.g.,square, trapezoid, parallelogram, or ellipse) may be used tocharacterize the perimeter of an overlay target boundary. For example, aset of structures of a given target may be arranged such that theiroutermost edges form an ellipsoidal or circular shaped target region.

Generally, the two dimensional shapes of the various pattern elements ofthe first structure and the second structure are not limited. As suchthe square shape of the pattern elements, as depicted in FIG. 3, shouldnot be interpreted as a limitation but merely an illustration. It isrecognized that a variety of pattern element shapes exist that mayproduce the 90 degree rotational invariance as required of the patternelements (e.g., 302 a through 312 b) of target structure 300. Forinstance, the pattern elements of target structure 300 may includepattern elements having a square shape, a cross shape, or a diamondshape, among others.

In another aspect, as illustrated in FIG. 3, the pattern elements of thefirst structure may be identical to the pattern elements of the secondstructure. For example, all of the pattern elements of target structure300 may have a square shape.

In another aspect, the pattern elements of the various structures of thetarget structure 300 may be different. For example, although not shown,the pattern elements 302 a and 302 b of the first structure may bedifferent from the pattern elements 304 a and 304 b of the secondstructure. For instance, the pattern elements 302 a and 302 b of thefirst structure may have a square shape, while the pattern elements 304a and 304 b of the second structure may have a ‘cross’ shape (notshown).

In another aspect, the shapes of the pattern elements within a singletarget structure (i.e., the first structure or the second structure) maybe uniform. More specifically, the pattern elements within a givenstructure may have an identical shape. For example, the pattern elements306 a and 306 b of the third target structure may both have a squareshape.

In another aspect, the shapes of the pattern elements within a givenstructure (i.e., the first structure or the second structure) may benon-uniform (not shown). More specifically, a given structure maycontain more than one pattern element shape. For example, the fourthstructure may include pattern element 308 a having a ‘cross’ shape (notshown) and a pattern element 308 b having a square shape. It should berecognized that there is no generalized limitation on the shape of thepattern elements of the target structures of overlay target 300,provided the shapes of the pattern elements and the orientation of thepattern elements results in the target structures having 180 degreerotational invariance about their common center of symmetry and eachpattern element of each target structure having 90 degree rotationalinvariance about its individual center of symmetry.

The pattern elements of the structures of overlay target 300 may bearranged according to various sets of spatial positions. For example,the pattern elements 302 a and 302 b of the first structure, the patternelements 304 a and 304 b of the second structure, the pattern elements306 a and 306 b of the third structure, the pattern elements 308 a and308 b of the fourth structure, the pattern elements 310 a and 310 b ofthe fifth structure, and the pattern elements 312 a and 312 b of thesixth structure may be arranged such that they form a periodic ornon-periodic pattern. For instance, as shown in FIG. 3, thetwo-dimensional arrangement of the pattern elements 302 a through 312 bforms a two-dimensional periodic array. It is contemplated herein that avariety of arrangements may be suitable for creating the 180 degreerotational invariance of the target 300.

FIG. 4 illustrates a top plan view of an overlay target 400, inaccordance with an alternate embodiment of the present invention.Applicant notes that unless otherwise noted the descriptive materialprovided above with respect to target 300 should be interpreted to applyto the remainder of the instant disclosure.

As in the target 300 described previously herein, the multilayer overlaytarget 400 may include three or more target structures, with each targetstructure including two or more pattern elements. For example, theoverlay target 400 may include six target structures, with each targetstructure containing four pattern elements. For example, as shown inFIG. 4, a first structure may include pattern elements 402 a, 402 b, 402c and 402 d, a second structure may contain pattern elements 404 a, 404b, 404 c, and 404 d, a third structure may contain pattern elements 406a, 406 b, 406 c, and 406 d, and so on. As in target 300, generallyspeaking, a given structure of target 400 (i.e., first, second, third,or up to an Nth structure) may contain from two pattern elements up toand including an Nth pattern element.

In another aspect of target 400, similar to the above described target300, each of the target structures of target 400 are designed such thateach is invariant to a 180 degree rotation about a common center ofsymmetry 110, resulting target 400 also being invariant to a 180 degreerotation. For example, as shown in FIG. 4, the pattern elements 402 aand 402 b of the first structure of target 400 are oriented diagonallyfrom the pattern elements 402 c and 402 d and arranged such that thefirst target structure is invariant to 180 degree rotation about itscenter of symmetry 110. It is noted, however, that the target structuresof target 400 are not invariant to a 90 degree rotation

Similar to target 300 above, target 400 may also be utilized in overlaymetrology between more than two layers. Resultantly, overlay metrologymeasurements may be performed utilizing any pair of the six targetstructures present in overlay target 400. Moreover, due to thecollocation of the center of symmetries 110 of each structure of target400, overlay metrology measurements may be acquired from all sixstructures in a single image grab.

In a further aspect of the present invention, for each target structure,the center of symmetry 110 for the set of pattern elements utilized forX-overlay measurements (e.g., 402 a and 402 d) is collocated with theset of pattern elements utilized for Y-overlay measurements (e.g., 402 band 402 c). It is recognized that a design such as this allows for thesimultaneous acquisition of X-overlay and Y-overlay data in a single“image grab.” As such, the move-acquire-measurement time as compared totraditional overlay targets is greatly reduced. Moreover, it is furtherrecognized that the design depicted in FIG. 4 may allow forcompatibility with presently existing metrology tool procedures andarchitectures.

In another aspect, the individual pattern elements of target 400 aredesigned such that each pattern element (e.g., 402 a-402 b, 404 a-404 band etc.) is invariant to a 180° rotation about the center of symmetry112 of the individual pattern element. In contrast to target 300, it isfurther noted that the individual pattern elements of target 400 are notinvariant to a 90° rotation about the center of symmetry 112 of theindividual pattern element. As such, a single pattern element (e.g., 402a) cannot be utilized to measure both X-overlay and Y-overlay. Thus,each individual pattern element may be utilized to measure eitherX-overlay or Y-overlay. For example, the target structures of target 400include pairs of pattern elements, one designated for X-overlay and onedesignated for Y-overlay. The shapes of the pattern elements depicted inFIG. 4 do not represent a limitation as it should be recognized thatthere exist a number of other pattern elements shapes having 180 degreerotational symmetry (but not 90 degree rotational symmetry) which aresuitable for implementation in the present invention.

In a general sense, any pattern element and target structure schemewhich produces 180 degree rotational symmetry (without producing 90degree rotational symmetry) for the target structures about the commoncenter of symmetry 110, while producing 180 degree rotational symmetry(without producing 90 degree rotational symmetry) for the individualpattern elements (e.g., 402 a through 412 d) about each pattern elementcenter of symmetry 112, may be suitable for implementation in thepresent invention. For this reason, the target structure and patternelement scheme depicted in FIG. 4 should be interpreted merely asillustrative and should not be considered limiting.

FIG. 5A illustrates a top plan view of an overlay target 500, inaccordance with an alternate embodiment of the present invention. As thepreviously described overlay targets, the multilayer target 500 mayinclude three or more target structures, with each target structureincluding two or more pattern elements. For example, as shown in FIG.5A, the overlay target 500 may include six target structures, with eachtarget structure containing four pattern elements. For example, as shownin FIG. 5A, a first structure may include pattern elements 502 a, 502 b,502 c and 502 d, a second structure may contain pattern elements 504 a,504 b, 504 c, and 504 d, and so on. Again, generally speaking, a givenstructure of target 500 (i.e., first, second, third, or up to an Nthstructure) may contain from two pattern elements up to and including anNth pattern element.

In contrast to targets 300 and 400, each of the target structures oftarget 500 are designed such that each is invariant to a 90 degreerotation about a common center of symmetry 110, resulting in target 500also being invariant to a 90 degree rotation. For example, as shown inFIG. 5A, the pattern elements 512 a, 512 b, 512 c, and 512 d of thesixth target structure of target 500 are arranged such that the sixthtarget structure is invariant to 90 degree rotation about its center ofsymmetry 110.

In another aspect, the individual pattern elements of target 500 aredesigned such that each pattern element (e.g., 502 a-502 d, 504 a-504 dand etc.) is invariant to a 180° rotation about the center of symmetryof the individual pattern element 112. Again, the pattern elements of500 are not invariant to a 90° rotation about the center of symmetry ofthe individual pattern element 112. Therefore, as in target 400, asingle pattern element (e.g., 502 a) cannot be utilized to measure bothX-overlay and Y-overlay. As such, each individual pattern element may beutilized to measure either X-overlay or Y-overlay. For example, thetarget structures of target 500 include two pairs of pattern elements,one pair (502 a and 502 c) designated for X-overlay measurement and onepair (502 b and 502 d) designated for Y-overlay measurement. Also as intarget 400, the shapes of the pattern elements depicted in FIG. 5 do notrepresent a limitation as it should be recognized that there exist anumber of other pattern elements shapes having 180 degree rotationalsymmetry (without producing 90 degree rotational symmetry) about anindividual center of symmetry 112 of the pattern element which aresuitable for implementation in the present invention.

In a general sense, any pattern element and target structure schemewhich produces 90 degree rotational symmetry for the target structuresabout the common center of symmetry 110, while producing 180 degreerotational symmetry (without producing 90 degree rotational symmetry)for the individual pattern elements (e.g., 502 a through 512 d) abouteach pattern element center of symmetry 112, may be suitable forimplementation in the present invention. For this reason, the targetstructure and pattern element scheme depicted in FIG. 5 should beinterpreted merely as illustrative and should not be consideredlimiting.

FIG. 5B illustrates a top plan view of an overlay target 501, inaccordance with an alternate embodiment of the present invention. As thepreviously described overlay targets, the multilayer target 501 mayinclude three or more target structures, with each target structureincluding two or more pattern elements. For example, as shown in FIG.5B, the overlay target 501 may include six target structures, with eachtarget structure containing four pattern elements. For example, as shownin FIG. 5B, a first structure may include pattern elements 514 a, 514 b,514 c and 514 d, a second structure may contain pattern elements 516 a,516 b, 516 c, and 516 d, a third structure may contain pattern elements518 a, 518 b, 518 c, and 518 d, and so on. Again, generally speaking, agiven structure of target 501 (i.e., first, second, third, or up to anNth structure) may contain from two pattern elements up to and includingan Nth pattern element.

In contrast to FIG. 5A, the overlay target 501 is designed to beinvariant to 180 degrees, but not invariant to 90 degrees. In thismanner, each of the target structures of target 501 are designed suchthat each is at least invariant to 180 degree rotation about a commoncenter of symmetry 110, resulting in target 501 also being invariant toa 180 degree rotation. For example, as shown in FIG. 5B, the patternelements 524 a, 524 b, 524 c, and 524 d of the sixth target structure oftarget 501 are arranged such that the sixth target structure isinvariant to 180 degree rotation (but not 90 degree rotation) about itscenter of symmetry 110. Applicant notes that each constituent targetstructure of overlay target 501 need not be limited to 180 degreerotational symmetry. For instance, as shown in FIG. 5B, it is noted thatthe arrangement of pattern elements 518 a, 518 b, 518 c, and 518 d forma 90 degree rotationally invariant target structure. As depicted in FIG.5B, however, the combination of the six target structures form anoverlay target 501 which lacks 90 degree rotational symmetry butpossesses 180 degree rotational symmetry since the remaining targetstructures lack 90 degree rotational symmetry.

In a general sense, any pattern element and target structure schemewhich produces 180 degree rotational symmetry for the target structuresabout the common center of symmetry 110, while producing 180 degreerotational symmetry for the individual pattern elements (e.g., 514 athrough 524 d) about each pattern element center of symmetry 112, may besuitable for implementation in the present invention. For this reason,the target structure and pattern element scheme depicted in FIG. 5Bshould be interpreted merely as illustrative and should not beconsidered limiting.

FIG. 6 illustrates a top plan view of overlay target 600, in accordancewith alternate embodiment of the present invention. It is recognizedthat one or more target structures of the various embodiments of theoverlay targets described previously herein may lack sufficient contrastsuitable for implementation in an overlay metrology measurement process.It is contemplated herein that one or more target structures of a givenoverlay target 600 may be enhanced by increasing the overall targetstructure surface area, thereby increasing the information content ofthe enhanced target structure. For example, the number of patternelements included in a given target structure may be determined by thelevel of contrast of the given target structure. For instance, as shownin FIG. 6, the first structure of target 600 may have lower contrastlevels than desirable. As such, the designer of the target may enhancethe contrast by including additional pattern elements to the targetstructure. In this manner, the first target structure of target 600includes four overall pattern elements 602 a, 602 b, 602 c, and 602 d,as opposed to the only two pattern elements in the remaining targetsstructures of the target 600.

It is also recognized that the additional pattern elements utilized toincrease contrast of a given target structure should be designed toadhere to the overall set of design rules for the given target. As such,the additional pattern elements should adhere to the symmetryrequirements placed on the overall target structure and individualpattern elements in a manner consistent with the above described targets300, 400, 500, and 501.

For example, as illustrated in FIG. 6, the pattern elements 602 a, 602b, 602 c, and 602 d maintain 180 degree rotational symmetry about thecenter of symmetry 110 of the overall target 600. Resultantly, thetarget 600 will maintain 180 degree rotational symmetry about the centerof symmetry 110 in a manner similar to targets 300, 400, and 501described previously herein. Furthermore, also as illustrated in FIG. 6,the pattern elements 602 a, 602 b, 602 c, and 602 d maintain 90 degreerotational symmetry about the center of symmetry of the individualpattern elements in a manner consistent with target 200 describedpreviously herein.

FIG. 7 illustrates a top plan view of overlay target 700, in accordancewith an alternate embodiment of the present invention. It iscontemplated herein that each target structure of target 700 may includethe number of pattern elements necessary to achieve adequate levels ofinformation content (i.e., contrast). In this manner, the informationcontent of one or more target structures may be satisfied by increasingthe overall target structure area of target structures lacking incontrast. For example, as shown in FIG. 7, the first structure, secondstructure, third structure and fourth structure of target 700 may havevarying degrees of information deficiencies. As such, the designer maytailor the number of pattern elements of each target structure to makeup for this deficiency. For example, the first structure, having thelowest level of contrast, may include twelve pattern elements 702 a, 702b, 702 c, 702 d, 702 e, 702 f, 702 g, 702 h, 702 i, 702 j, 702 k, and702 l. Likewise, the second and third structures may have a similarlevel of contrast needs, each including eight overall pattern elements.The second structure includes 704 a, 704 b, 704 c, 704 d, 704 e, 704 f,704 g, and 704 h, while the third structure includes 706 a, 706 b, 706c, and 706 d. In contrast, the fourth target structure of target 700 mayrequire little contrast enhancement or may have surplus informationcontent. In this manner, surface area normally designated for the fourthsurface structure may be reallocated to one of the other targetstructures in order to build up contrast in those lacking targetstructures while maintaining the overall surface area requirements forthe overlay target 700. For example, the fourth target structure mayinclude only 4 target pattern elements 708 a, 708 b, 708 c, and 708 d.

It is also recognized that the additional pattern elements utilized toincrease contrast of the target structures of overlay target 700 shouldbe designed to adhere to the overall set of design rules for the giventarget. As such, the additional pattern elements should adhere to thesymmetry requirements placed on the overall target structure andindividual pattern elements in a manner consistent with the abovedescribed targets 400, 500, and 501.

For example, as illustrated in FIG. 7, the pattern elements 704 a . . .704 h of the second target structure maintain 90 degree rotationalsymmetry about the center of symmetry 110 of the overall target 700,while pattern elements 708 a . . . 708 d of the fourth target structurepossess 180 degree rotational symmetry about the center of symmetry 110.Resultantly, the target 700 will maintain at least 180 degree rotationalsymmetry about the center of symmetry 110 in a manner similar to targets400 and 501 described previously herein. It is further recognized thatthe above described utilization of additional pattern elements may alsobe implemented such that the overlay target possesses 90 degreerotational symmetry similar to target 500 illustrated in FIG. 5A.

Furthermore, also as illustrated in FIG. 7, the individual patternelements 702 a . . . 702 l, 704 a . . . 704 h, 706 a . . . 706 h, and708 a . . . 708 d each are 180 degree rotationally symmetric about thecenter of symmetry of each individual pattern element in a mannerconsistent with target 400, 500, and 501 described previously herein.

FIG. 8 illustrates a top plan view of overlay target 800 in the presenceof dummy fill 801, in accordance with an alternate embodiment of thepresent invention. It should be recognized that the overlay targets 400,500, and 501, wherein X-overlay and Y-overlay measurements are performedutilizing different pattern elements, allow for overlay metrologymeasurement processes in the presence of dummy fill 801. For example,FIG. 8 depicts an overlay target 800 implemented in the presence ofdummy fill 801. For instance, overlay target 800 includes six targetstructures, with each target structure including four pattern elements.In this manner, the first structure includes pattern elements 802 a . .. 802 d, the second structure includes pattern elements 804 a . . . 804d, the third structure includes pattern elements 806 a . . . 806 d, thefourth structure includes pattern elements 808 a . . . 808 d, the fifthstructure includes pattern elements 810 a . . . 810 d, and the sixthstructure includes pattern elements 812 a . . . 812 d. Moreover, it ispointed out that in the example of FIG. 8 two of the pattern elements ofeach structure are designated for X-overlay measurement (e.g., 802 a,806 a, or 810 a), while the remaining two pattern elements of eachtarget structure are designated for Y-overlay measurement (e.g., 812 d,808 d, or 804 d).

In a further embodiment, the pattern elements (e.g., 802 a . . . 812 d)of target 800 each include a plurality of sub-elements 803. For example,as illustrated in FIG. 8, each pattern element (e.g., 802 a . . . 812 d)may include three parallel thin rectangular shaped and periodicallyspaced sub-elements 803. It should be noted that the shape andarrangement of the sub-elements 803 depicted in FIG. 8 does notrepresent a limitation but rather should be interpreted as illustrative.

It is further recognized that the dummy fill 801 may consist of aperiodic grating structure printed above or below the overlay target 800as illustrated by the FIG. 8.

In a further embodiment, the sub-elements 803 of each pattern element(e.g., 802 a . . . 812 d) of each structure may be aligned orthogonallywith the grating structure of the dummy fill 801 structure. In thisregard, the lines of the dummy fill 801 run perpendicularly to the linesof the sub-element 803 structure. Applicant notes that by aligning thesub-elements 803 of the pattern elements (e.g., 802 a . . . 812 d)orthogonally to the dummy fill structure 801 mitigates the risk ofcontamination of the metrology signal of a given overlay target withinformation from the underlying dummy fill structure 801.

As in targets 400 and 501 described previously herein, it is furtherrecognized that the overlay target 800 possesses 180 degree rotationalsymmetry about the common center of symmetry of the constituent targetstructures of the target, while the individual pattern elements (e.g.,802 a . . . 812 d) of the target 800 possess 180 degree rotationalsymmetric about the center of symmetry of each individual patternelement.

In a further embodiment, the periodicity of the sub-elements 803 of thepattern elements (e.g., 802 a . . . 812 d), the dummy fill structure801, or both may consist of a resolution below that which is suitablefor the implementing metrology system. In particular, the 1st and −1stdiffraction orders may fall outside the aperture of the objective of theimaging system of the metrology system. It is recognized herein thatthis feature is particularly advantageous in the case of the dummy fillstructure as it further mitigates the risk of contamination of themetrology signal of the target 800 with a signal from the dummy fillpattern 801.

FIG. 9 illustrates a top plan view of overlay target 900 in the presenceof dummy fill 801, in accordance with an alternate embodiment of thepresent invention. Target 900 is similar to target 800 in that itpossesses identical symmetry requirements as well as orthogonal patternelement and dummy fill alignment. Target 900, however, illustrates asquare dimensioned target suitable for implementation in a metrologyprocess.

FIG. 10 illustrates a top plan view of overlay target 1000 in thepresence of dummy fill 801, in accordance with an alternate embodimentof the present invention. Target 1000 is similar to target 800 in thatit possesses identical symmetry requirements as well as orthogonalpattern element and dummy fill alignment. Target 1000, however,illustrates the implementation of contrast enhancement as describedpreviously herein with respect to FIGS. 6 and 7. Furthermore, FIG. 10illustrates an acquisition mark 1001 located at the center of theoverlay target 1000. The acquisition mark 1001 may be utilized toidentify the approximate position of the center of the target in orderto position the target in the center of the field of view (FOV) of thegiven metrology tool.

Referring generally to FIGS. 11 and 12, the systems 1100 and 1200suitable for contrast enhancement are described in accordance with thepresent invention. It is contemplated herein that systems 1100 and 1200of the present invention may enable the implementation of the variousmulti-layer targets described previously herein. One limitationassociated with the multi-layer targets of the present inventionincludes the potential for lack of information content (i.e., contrastlevel) associated with their small measurement structures. The systems1100 and 1200 are directed at providing enhanced contrast levels tocounteract the presence of low contrast in one or more target structuresof the various multi-layer targets of the present invention. The system1100 is directed at the utilization of structured illumination in orderto enhance the contrast level associated with one or more measurementstructures associated with the target structures of the multi-layertargets of the present invention. Moreover, the system 1200 is directedat the utilization of cross-polarization in order to enhance thecontrast level associated with one or more measurement structuresassociated with the target structures of the multi-layer targets of thepresent invention.

It is contemplated herein that the systems 1100 and 1200 of the presentinvention may consist (but not required to consist) of adapting orreconfiguring presently existing optical metrology systems. Forinstance, the present invention may consist of adapting the KLA-TencorArcher 100 overlay control system. For example, in the case of system1200, a first linear polarizer may be inserted into an illumination pathof a traditional system (e.g., Archer 100 system), while a second linearpolarizer is placed within the imaging path of the traditional system.In the case of system 1100, an aperture may be inserted at a pupil planeof an illumination path of a traditional system (e.g., Archer 100system). It should be recognized that the present invention is notlimited to an adaptation of an Archer 100 system, but rather thedescription above should be interpreted merely as an illustration. It isanticipated that the present invention may be extended to a wide varietyof microscopy and overlay metrology systems.

Referring now to FIG. 11, the system 1100 suitable for contrastenhancement of a multi-layer overlay metrology target may include anillumination source 1102, an aperture 1104, a beam splitter 1108, and adetector 1110 configured to receive light reflected from one or morespecimens 1114 (e.g., one or more wafers of a wafer lot).

The illumination source 1102 of the system 1100 may include anyillumination source known in the art. In one embodiment, theillumination source 1102 may include a broadband light source (e.g.,white light source). For example, the illumination source 1102 mayinclude, but is not limited to, a halogen light source (HLS). Forinstance, the halogen light source may include, but is not limited to, atungsten based halogen lamp. In another example, the illumination source1102 may include a Xenon arc lamp.

In another aspect of the present invention, the beam splitter 1108 ofthe system 1100 may split the light beam emanating from an illuminationsource 1102, after passing through the aperture, into two paths: anobject path 1112 and a reference path 1113. In this sense, the objectpath 1112 and the reference path 113 of the system 100 may form aportion of a two beam interference optical system. For example, the beamsplitter 1108 may direct a first portion of the beam of light from theillumination path 1115 along the object path 1112, while allowing asecond portion of the beam of light from the illumination path 115 to betransmitted along the reference path 1113. More specifically, the beamsplitter 1108 may direct a portion of the light emanating from theillumination source 1102, after passing through the aperture 1104, tothe surface of the specimen 1114 (e.g., via object path 1112) disposedon the specimen stage 1118. Moreover, the beam splitter 1108 maytransmit a second portion of the light emanating from the illuminationsource 1102 to the components of the reference path 1113. For instance,the beam splitter 1108 may transmit a portion of light from theillumination path 1115 along the reference path 1113 to a referencemirror (not shown). It should be recognized by those skilled in the artthat any beam splitter known in the art is suitable for implementationas the 1 beam splitter 1108 of the present invention.

It should be apparent to those skilled in the art that the referencepath 1113 may include, but is not limited to, a reference mirror, areference objective, and a shutter configured to selectively block thereference path 1113. In a general sense, a two-beam interference opticalsystem may be configured as a Linnik interferometer. Linnikinterferometry is described generally in U.S. Pat. No. 4,818,110, issuedon Apr. 4, 1989, and U.S. Pat. No. 6,172,349, issued on Jan. 9, 2001,which are incorporated herein by reference.

In another embodiment, the system 1100 may include a main objective lens1109. The main objective lens 1109 may aid in directing light along theobject path 1112 to the surface of the specimen 1114 disposed on thespecimen stage 1118. For example, the beam splitter 1108 may direct aportion of the light beam 1115 emanating from the illumination source1102, after passing through the aperture 1104, along the object path1112. Following the splitting process by the beam splitter 1108, themain objective lens 1109 may focus light from the object path 1112,which is collinear with the primary optical axis 1107, onto the surfaceof the specimen 1114. In a general sense, any objective lens known inthe art may be suitable for implementation as the main objective lens1109 of the present invention.

Further, a portion of the light impinging on the surface of the specimen1114 may be reflected by the specimen 1114 and directed along theprimary optical axis 1107 via the objective 1109 and the beam splitter1108 toward the detector 1110. It should be further recognized thatintermediate optics devices such as intermediate lenses, additional beamsplitters (e.g., a beam splitter configured to split off a portion oflight to a focusing system), and imaging lenses 1106 may be placedbetween the objective 1109 and the imaging plane of the detector 1110.

In another aspect of the present invention, the detector 1110 of thesystem 1100 may be disposed along the primary optical axis 1107 of thesystem 1100. In this regard, the camera 1110 may be arranged to collectimagery data from the surface of the specimen 1114. For example, in ageneral sense, after reflecting from the surface of the specimen 1114,light may travel along the primary optical axis 1107 to the image planeof the detector 1110 via the main objective 1109 and the beam splitter1108. It is recognized that any detector system known in the art issuitable for implementation in the present invention. For example, thedetector 1110 may include a charge coupled device (CCD) based camerasystem. By way of another example, the detector 1110 may include a timedelay integration (TDI)-CCD based camera system. In a further aspect,the detector 1110 may be communicatively coupled with a computer system(not shown). In this regard, digitized imagery data may be transmittedfrom the detector 1110 to the computer system via a signal, such as awireline signal (e.g., copper line, fiber optic cable, and the like) ora wireless signal (e.g., wireless RF signal).

While the above description describes the detector 1110 as being locatedalong the primary optical axis 1107 of the system 1100, thischaracteristic should not be interpreted as a requirement. It iscontemplated herein that the detector 1110 may reside along anadditional optical axis of the system 1100. For example, in a generalsense, one or more additional beam splitters may be utilized to divert aportion of light reflected from the surface of the specimen 1114 andtraveling along the object path 1112 onto an additional optical axis,which non-parallel to the object path 1112. The camera 1110 may bearranged such that light traveling along the additional optical axisimpinges the image plane of the camera 1110.

In one aspect of the present invention the aperture 1104 may be positionat a pupil plane of the illumination path 1115. In this regard, theaperture 1104 may be configured to have a well-defined shape in order toselect an predetermined illumination angle of the illumination emanatingfrom the illumination source 1102. The illumination angle is selected soas to achieve a selected contrast level at an imaging plane of thedetector 1110.

In one embodiment, the aperture may have a geometric shape or acombination of geometric shapes. For example, the aperture may have an‘X’ shape or a ‘cross’ shape. In another example, the aperture may havea ring shape. It is further recognized herein that these shapes may beachieved via diffractive optical elements.

In another embodiment, the illumination path may include a plurality ofapertures. In this regard, one of the plurality of apertures may beselected during recipe training in order to optimize the contrast levelfor a specific stack and target design. It is recognized herein thatthis may be done utilizing a trial and error method. In anotherembodiment, the aperture 1104 may include a tunable aperture. Forexample, the aperture 1104 may consist of a tunable aperture that may beprogrammed by a user in order to produce a plurality of selectableillumination structures. In this regard, a programmed tunable aperturemay be tuned in a manner to optimize the contrast for a specific stackor target design. For instance, the tunable aperture may include, but isnot limited to, a micro mirror array.

Referring now to FIG. 12, the system 1200 suitable for contrastenhancement of a multi-layer overlay metrology target may include anillumination source 1202, a first polarizer 1204, a beam splitter 1206,a second polarizer 1208 and a detector 1210 configured to receive lightreflected from one or more specimens 1212 (e.g., one or more wafers of awafer lot).

It is recognized herein that the illumination source 1202, the beamsplitter 1206, the detector of 1210, the specimen stage 1214, and thereference path 1216 are similar to the illumination source 1102, thebeam splitter 1108, the detector of 1110, the specimen stage 1118, andthe reference path of 1113 of system 1100. As such, the description ofsystem 1100 should be interpreted to extend to system 1200 except whereotherwise noted.

In one aspect, the first polarizer 1204 is arranged to polarize lightemanating from the illumination source 1202. For example, the first 1204may be disposed along an illumination path 1205 such that lightemanating from the illumination source 1202 may be polarized by thefirst polarizer 1204.

In another aspect, the second polarizer 1208 may be arranged to serve asan analyzer for light reflected from the specimen 1212. In this regard,the first polarizer 1204 and the second polarizer 1208 may configured besuch that the amount of light reflected from unpatterned parts of thespecimen 1212 or from periodic unresolved patterns of the specimen 1212that reaches the imaging plane of the detector 1210 is minimized. In oneembodiment, the first polarizer 1204 and the second polarizer 1208 mayboth include linear polarizers. In the case of linear polarizers, thefirst polarizer 1204 and the second polarizer 1208 may be arranged suchthat their polarizing axes are substantially perpendicular to oneanother. As a result of this configuration, the majority of reflectedlight reaching the imaging plane of the detector 1210 consists of lightreflected from patterns of the specimen 1212 resolved by the metrologytool, enhancing the contrast significantly. In further another, thefirst polarizer 1204 may include a polarizer configured to transmit onlyradially polarized light, while the second polarizer is configured totransmit only azimuthally polarized light.

It should be further recognized that the signal from unpatternedportions of the specimen 1212 may be minimized in a variety of othermanners. For example, it is recognized herein that a combination ofwave-plates and polarizers may be implemented to achieve the resultsillustrated above. For instance, a first polarizer 1204 and firstquarter-wave plate (not shown) oriented at 45 degrees with respect tothe first polarizer may be positioned in the illumination path 1205,while a second polarizer 1208 and a second quarter-wave plate (notshown) oriented at 45 degree with respect to the second polarizer may bepositioned along the imaging path 1209. Those skilled in the art willrecognize that this arrangement may lead to a minimization of the amountlight reflected from unpatterned portions of the specimen 1212 whichreaches the imaging plane of the detector 1210.

It is further recognized that any combination of polarizers andwave-plates (e.g., half-wave plate) which creates the cross-polarizationeffect as described above may be suitable for implementation in thepresent invention.

It is further contemplated herein that the systems 1100 and 1200 may beutilized in combination to improve the level of contrast. In thisregard, the present invention may be utilized to ensure a low level ofintensity at a point of symmetry of the target. It is recognized hereinthat the combination of structured illumination and cross-polarizationaspects of the present invention may be implemented utilizing theillumination pupils illustrated in FIG. 13. For example, a suitableillumination pupil may have a cross-shape 1302, a vertical line shape1304 (e.g., Y-direction), or a horizontal line shape 1306 (e.g.,X-direction). Moreover, the illumination pupils 1302, 1304, and 1306 maybe implemented in combination with an illumination polarizer and animaging polarizer. In a first embodiment, the pupils 1302-1306 may beimplemented in concert with a X-polarizer disposed within theillumination path (e.g., 1115 or 1205) of the system and a Y-polarizerdisposed within the imaging path (e.g., 1107 or 1207) of the system. Ina second embodiment, the pupils 1302-1306 may be implement in concertwith a Y-polarizer disposed within the illumination path of the systemand a X-polarizer disposed within the imaging path of the system.

All of the system and methods described herein may include storingresults of one or more steps of the method embodiments in a storagemedium. The results may include any of the results described herein andmay be stored in any manner known in the art. The storage medium mayinclude any storage medium described herein or any other suitablestorage medium known in the art. After the results have been stored, theresults can be accessed in the storage medium and used by any of themethod or system embodiments described herein, formatted for display toa user, used by another software module, method, or system, etc.Furthermore, the results may be stored “permanently,”“semi-permanently,” temporarily, or for some period of time. Forexample, the storage medium may be random access memory (RAM), and theresults may not necessarily persist indefinitely in the storage medium.

Those having skill in the art will appreciate that there are variousvehicles by which processes and/or systems and/or other technologiesdescribed herein can be effected (e.g., hardware, software, and/orfirmware), and that the preferred vehicle will vary with the context inwhich the processes and/or systems and/or other technologies aredeployed. For example, if an implementer determines that speed andaccuracy are paramount, the implementer may opt for a mainly hardwareand/or firmware vehicle; alternatively, if flexibility is paramount, theimplementer may opt for a mainly software implementation; or, yet againalternatively, the implementer may opt for some combination of hardware,software, and/or firmware. Hence, there are several possible vehicles bywhich the processes and/or devices and/or other technologies describedherein may be effected, none of which is inherently superior to theother in that any vehicle to be utilized is a choice dependent upon thecontext in which the vehicle will be deployed and the specific concerns(e.g., speed, flexibility, or predictability) of the implementer, any ofwhich may vary. Those skilled in the art will recognize that opticalaspects of implementations will typically employ optically-orientedhardware, software, and or firmware.

Those skilled in the art will recognize that it is common within the artto describe devices and/or processes in the fashion set forth herein,and thereafter use engineering practices to integrate such describeddevices and/or processes into data processing systems. That is, at leasta portion of the devices and/or processes described herein can beintegrated into a data processing system via a reasonable amount ofexperimentation. Those having skill in the art will recognize that atypical data processing system generally includes one or more of asystem unit housing, a video display device, a memory such as volatileand non-volatile memory, processors such as microprocessors and digitalsignal processors, computational entities such as operating systems,drivers, graphical user interfaces, and applications programs, one ormore interaction devices, such as a touch pad or screen, and/or controlsystems including feedback loops and control motors (e.g., feedback forsensing position and/or velocity; control motors for moving and/oradjusting components and/or quantities). A typical data processingsystem may be implemented utilizing any suitable commercially availablecomponents, such as those typically found in datacomputing/communication and/or network computing/communication systems.

The herein described subject matter sometimes illustrates differentcomponents contained within, or connected with, different othercomponents. It is to be understood that such depicted architectures aremerely exemplary, and that in fact many other architectures can beimplemented which achieve the same functionality. In a conceptual sense,any arrangement of components to achieve the same functionality iseffectively “associated” such that the desired functionality isachieved. Hence, any two components herein combined to achieve aparticular functionality can be seen as “associated with” each othersuch that the desired functionality is achieved, irrespective ofarchitectures or intermedial components. Likewise, any two components soassociated can also be viewed as being “connected”, or “coupled”, toeach other to achieve the desired functionality, and any two componentscapable of being so associated can also be viewed as being “couplable”,to each other to achieve the desired functionality. Specific examples ofcouplable include but are not limited to physically mateable and/orphysically interacting components and/or wirelessly interactable and/orwirelessly interacting components and/or logically interacting and/orlogically interactable components.

While particular aspects of the present subject matter described hereinhave been shown and described, it will be apparent to those skilled inthe art that, based upon the teachings herein, changes and modificationsmay be made without departing from the subject matter described hereinand its broader aspects and, therefore, the appended claims are toencompass within their scope all such changes and modifications as arewithin the true spirit and scope of the subject matter described herein.

Although particular embodiments of this invention have been illustrated,it is apparent that various modifications and embodiments of theinvention may be made by those skilled in the art without departing fromthe scope and spirit of the foregoing disclosure. Accordingly, the scopeof the invention should be limited only by the claims appended hereto.

It is believed that the present disclosure and many of its attendantadvantages will be understood by the foregoing description, and it willbe apparent that various changes may be made in the form, constructionand arrangement of the components without departing from the disclosedsubject matter or without sacrificing all of its material advantages.The form described is merely explanatory, and it is the intention of thefollowing claims to encompass and include such changes.

Furthermore, it is to be understood that the invention is defined by theappended claims.

What is claimed:
 1. A system for measuring overlay from a multilayeroverlay target comprising: an illumination source configured toilluminate a plurality of target structures disposed on one or moreprocess layers of a semiconductor device, the plurality of targetstructures including three or more target structures, the three or moretarget structures including a first target structure, a second targetstructure and at least a third target structure, wherein at least someof the target structures include a set of two or more pattern elements,wherein the three or more target structures are configured to share acommon center of symmetry upon alignment of the three or more targetstructures, wherein at least one of the target structures is invariantto 90 degree rotation about the common center of symmetry, wherein thefirst target structure is disposed in a first process layer, wherein thesecond target structure is disposed in a second process layer differentfrom the first process layer, wherein at least the third targetstructure is disposed in at least a third process layer, the at least athird process layer different from the first process layer and thesecond process layer, wherein a first particular pattern element of theset of two or more pattern elements includes three or more sub-elementsand at least a second particular pattern element of the set of two ormore pattern elements includes three or more sub-elements, wherein atleast one of the three or more sub-elements of the first particularpattern element or the three or more sub-elements of the at least thesecond particular pattern element comprise a set of three or moreparallel line structures aligned along a selected direction anddistributed along a direction orthogonal to the selected direction; adetector configured to collect light reflected from the plurality oftarget structures; and one or more processors configured to execute aset of program instructions maintained on a non-transitory memorymedium, the set of program instructions configured to cause the one ormore processors to determine overlay error between two or morestructures based on the collected light from the plurality of targets.2. The system of claim 1, wherein a first pattern element of the set oftwo or more pattern elements is suitable for overlay metrologymeasurements in a first direction and a second pattern element of theset of two or more pattern elements is suitable for overlay metrologymeasurements in a second direction different from the first direction.3. The system of claim 1, wherein a set of pattern elements suitable foroverlay metrology measurements in a first direction and a second set ofpattern elements suitable for overlay metrology measurements in a seconddirection different from the first direction have a common center ofsymmetry.
 4. The system of claim 1, wherein the set of two or morepattern elements of each target structure are printed above or below alayer of dummy fill.
 5. The system of claim 1, wherein some of theplurality of target structures include an additional set of patternelements in order to enhance contrast of the some of the plurality oftarget structures.
 6. The system of claim 1, wherein at least some ofthe two or more pattern elements of each target structure are invariantto 180 degree rotation about an individual center of symmetry andvariant to 90 degree rotation about the individual center of symmetry.7. The system of claim 1, wherein the first target structure isinvariant to 90 degrees rotation about the common center of symmetry,wherein the second target structure and at least the third targetstructure are invariant to 180 degrees rotation about the common centerof symmetry and variant to 90 degrees rotation about the common centerof symmetry.
 8. The system of claim 1, wherein the first targetstructure is configured to measure overlay in a first direction and asecond direction perpendicular to the first direction, wherein thesecond target structure is configured to measure overlay in the firstdirection and the third target structure is configured to measureoverlay in the second direction.
 9. The system of claim 1, wherein thefirst target structure is invariant to 90 degree rotation about a commoncenter of symmetry, wherein at least one of the second target structureor the third target structure is invariant to 180 degree rotation aboutthe common center of symmetry and variant to 90 degree rotation aboutthe common center of symmetry, wherein each of the two or more patternelements of each target structure is invariant to 180 degree rotationabout an individual center of symmetry and variant to 90 degree rotationabout the individual center of symmetry.
 10. The system of claim 1,wherein the three or more sub-elements are arranged parallel to aspacing of the at least one of the pattern elements.
 11. The system ofclaim 1, wherein the three or more sub-elements are arrangedperpendicular to a spacing of the at least one of the pattern elements.12. The system of claim 1, wherein the three or more sub-elements arearranged parallel to a first spacing of the at least one of the patternelements and perpendicular to a second spacing of the at least one ofthe pattern elements.
 13. The system of claim 1, wherein a spacingassociated with the three or more sub-elements is smaller than a spacingbetween the two or more of the pattern elements.
 14. The system of claim1, wherein at least some of the two or more pattern elements have anindividual center of symmetry.
 15. A system for measuring overlay from amultilayer overlay target comprising: an illumination source configuredto illuminate a plurality of target structures disposed on one or moreprocess layers of a semiconductor device, the plurality of targetstructures including three or more target structures, the three or moretarget structures including a first target structure, a second targetstructure and at least a third target structure, wherein at least someof the target structures include a set of two or more pattern elements,wherein the three or more target structures are configured to share acommon center of symmetry upon alignment of the three or more targetstructures, wherein the first target structure is invariant to a 90degree rotation about the common center of symmetry, the second targetstructure is invariant to a 90 degree rotation about the common centerof symmetry, and at least the third target structure is invariant to a90 degree rotation about the common center of symmetry, wherein thefirst target structure is disposed in a first process layer, wherein thesecond target structure is disposed in a second process layer differentfrom the first process layer, wherein at least the third targetstructure is disposed in at least a third process layer, the at least athird process layer different from the first process layer and thesecond process layer, wherein at least some of the two or more patternelements have an individual center of symmetry different from the commoncenter of symmetry of the three or more target structures, wherein atleast a portion of a pattern element of the first target structureoverlaps with at least a portion of a pattern element of at least one ofthe second target structure or the at least a third target structure; adetector configured to collect light reflected from the plurality oftarget structures; and one or more processors configured to execute aset of program instructions maintained on a non-transitory memorymedium, the set of program instructions configured to cause the one ormore processors to determine overlay error between two or morestructures based on the collected light from the plurality of targets.16. The system of claim 15, wherein the three or more target structuresinclude at least a fourth target structure.
 17. The system of claim 16,wherein at least the fourth target structure is invariant to a 90 degreerotation about the common center of symmetry.
 18. The system of claim17, wherein the first target structure, the second target structure, thethird target structure and the fourth target structure are configured tomeasure overlay in a first direction and a second directionperpendicular to the first direction.
 19. The system of claim 15,wherein one or more pattern elements of the first target structureoverlaps with one or more pattern elements of at least one of the secondtarget structure or the at least a third target structure.
 20. A systemfor measuring overlay from a multilayer overlay target comprising: anillumination source configured to illuminate a plurality of targetstructures disposed on one or more process layers of a semiconductordevice, the plurality of target structures including three or moretarget structures, the three or more target structures including a firsttarget structure, a second target structure and at least a third targetstructure, wherein at least some of the target structures include a setof two or more pattern elements, wherein at least some of the two ormore pattern elements are reflection invariant, wherein at least some ofthe two or more pattern elements of each target structure are variant to90 degree rotation about an individual center of symmetry, wherein thetwo or more pattern elements of the multi-layer overlay target arespatially separated from one another, wherein the three or more targetstructures are configured to share a common center of symmetry uponalignment of the three or more target structures, wherein the firsttarget structure is disposed in a first process layer, wherein thesecond target structure is disposed in a second process layer differentfrom the first process layer, wherein at least the third targetstructure is disposed in at least a third process layer, the at least athird process layer different from the first process layer and thesecond process layer; a detector configured to collect light reflectedfrom the plurality of target structures; and one or more processorsconfigured to execute a set of program instructions maintained on anon-transitory memory medium, the set of program instructions configuredto cause the one or more processors to determine overlay error betweentwo or more structures based on the collected light from the pluralityof targets.
 21. A system for measuring overlay from a multilayer overlaytarget comprising: an illumination source configured to illuminate aplurality of target structures disposed on one or more process layers ofa semiconductor device, the plurality of target structures includingthree or more target structures, the three or more target structuresincluding a first target structure, a second target structure and atleast a third target structure, wherein at least some of the targetstructures include a set of two or more pattern elements, wherein thetwo or more pattern elements of the multi-layer overlay target arespatially separated from one another, wherein at least some of thetarget structures are reflection invariant, wherein at least some of thetwo or more pattern elements of each target structure are reflectioninvariant and variant to 90 degree rotation about an individual centerof symmetry, wherein the three or more target structures are configuredto share a common center of symmetry upon alignment of the three or moretarget structures, wherein the first target structure is disposed in afirst process layer, wherein the second target structure is disposed ina second process layer different from the first process layer, whereinat least the third target structure is disposed in at least a thirdprocess layer, the at least a third process layer different from thefirst process layer and the second process layer; a detector configuredto collect light reflected from the plurality of target structures; andone or more processors configured to execute a set of programinstructions maintained on a non-transitory memory medium, the set ofprogram instructions configured to cause the one or more processors todetermine overlay error between two or more structures based on thecollected light from the plurality of targets.
 22. A system formeasuring overlay from a multilayer overlay target comprising: anillumination source configured to illuminate a plurality of targetstructures disposed on one or more process layers of a semiconductordevice, the plurality of target structures including four or more targetstructures, the four or more target structures including a first targetstructure, a second target structure, a third target structure and atleast a fourth target structure, wherein at least some of the targetstructures include a set of two or more pattern elements, wherein thetwo or more pattern elements of the multi-layer overlay target arespatially separated from one another, wherein at least one of the two ormore pattern elements are variant to 90 degree rotation about individualcenters of symmetry, wherein a location of a center of symmetry of eachof the four or more target structures is indicative of an overlayalignment of the four or more target structures, wherein the four ormore target structures are configured to share a common center ofsymmetry upon alignment of the four or more target structures, whereinthe first target structure, the second target structure, the thirdtarget structure and the fourth target structure are invariant to a 180degree rotation about the common center of symmetry and variant to a 90degree rotation about the common center of symmetry, wherein the firsttarget structure is disposed in the first process layer, wherein thesecond target structure is disposed in the second process layerdifferent from the first process layer, wherein the third targetstructure is disposed in a third process layer different from the firstprocess layer and the second process layer, wherein the at least thefourth target structure is disposed within the fourth process layerdifferent from the first process layer, the second process layer and thethird process layer; a detector configured to collect light reflectedfrom the plurality of target structures; and one or more processorsconfigured to execute a set of program instructions maintained on anon-transitory memory medium, the set of program instructions configuredto cause the one or more processors to determine overlay error betweentwo or more structures based on the collected light from the pluralityof targets.
 23. The system of claim 22, wherein at least one of thefirst target structure or second target structure are configured tomeasure overlay in a first direction, wherein at least one of the thirdtarget structure or the fourth target structure are configured tomeasure overlay in a second direction perpendicular to the firstdirection.
 24. The system of claim 22, wherein at least some of the twoor more pattern elements of each target structure are invariant to 180degree rotation about an individual center of symmetry and variant to 90degree rotation about the individual center of symmetry, wherein theindividual center of symmetry of the at least some of the two or morepattern elements is different from the common center of symmetry of thefour or more target structures.